Pulmonary surfactants are complex lipid and protein compositions that can be extracted from animals, purified and used to treat neonatal respiratory distress syndrome. Animal-derived surfactants are difficult to purify, limited by the scale to which they can be manufactured and have the potential to cause immunogenic reactions on repeat use, and thus are not indicated for use in other clinical indications. To capitalize on the potential use of surfactants in a range of pulmonary disease states requires the availability of a functional surfactant that can both be manufactured at the scale required and which inherently has the flexibility to be modified for use with specific delivery systems and for specific disease states. One such surfactant under development is synthetically produced and has a composition of dipalmitoyl phosphatidylcholine (DPPC), or 1-palmitoyl 2-oleoyl phosphatidylglycerol (POPG), palmitic acid (PA) and an engineered mimic of surfactant protein B, called sinapultide (KL4 or KL4) dispersed within an isotonic aqueous Tris-saline buffer of pH 7.7. The composition is currently under evaluation for use in RDS, meconium aspiration syndrome and in acute respiratory distress syndrome at various concentrations. The composition can also be modified for use by reducing, increasing or substituting one or more of the components.
Previous work has shown that formulations having reduced concentrations of either palmitic acid (PA) or cetyl alcohol (CA), relative to that of 30 mg/ml Surfaxin® (i.e., 4.05 mg/ml PA), exhibit lower viscosity when compared side by side with Surfaxin®. However, this is at the expense of loss of functional surface tension activity as the content of the palmitic acid or cetyl alcohol is reduced as measured by in vitro techniques. However, by using DPPG instead of POPG and adding cholesterol, palmitic acid could be completely omitted from the formulation with the resulting compositions exhibiting low viscosity while retaining good surface activity.
Recently it was found that cholesterol at low concentrations contribute significantly to the termination of phase separation during compression of interfacial films of the pulmonary surfactant (Discher et al., 1999, Biochemistry 38:374-83). Other studies suggested that at low concentration cholesterol contributes to the elastic response of the neighboring lipids in the lipid bilayer. This elastic response is expressed by a tendency of the surrounding lipids to adapt to the hydrophobic shape of cholesterol (Kessel et al., 2001, Biophys J. 81:643-58). In fact, cholesterol can modulate physical properties of lipid bilayers to induce liquid phase coexistence and corresponding domain formation (Radhakrishnan and McConnell, 1999, Biophys J. 77:1507-17).
Due to these findings, and the existence of cholesterol in natural lung surfactant this report is related to the inclusion of various concentrations of cholesterol, 1-30 mol %, focusing on 2-15 mol %, to KL4-containing formulations to improve their properties. In particular (but not limited to) it can help in aerosolization of KL4-containing formulations by decreasing the viscosity of the formulations and otherwise modifying the structure of the concentrated lipid-based dispersions, such that an increase in the amount of surfactant that can be aerosolized is achieved. In addition, inclusion of cholesterol with saturated lipids in the surfactant formulation could facilitate and enhance an increase in lateral stability of the monolayer essential for alveolar expansion. Moreover, since the transition temperature of DPPC is close to body temperature (41° C.) cholesterol will “soften” the bilayer, increase its permeability and eliminate phase-separation during alveolar compression and expansion. Furthermore, cholesterol will contribute to membrane perturbation effects, thus will decrease the energy of interfacial tension and lower line tension (as in the critical point, when the characteristics of two phases become similar). Similarly it may improve lipid-peptide interaction, particularly KL4-DPPC/DPPG interactions.
The current standard method of delivery does not instantaneously deliver the surfactant over the surface of the airways and alveoli. The procedure involves the introduction of a bolus of surfactant to a patient who has been intubated (an invasive procedure). In order to distribute throughout the lungs, the surfactant must be aspirated deeper into the lungs during breathing maneuvers while simultaneously flowing and spreading across the lung surfaces. Accordingly, a formulation that can be effectively delivered as an aerosol in sufficient quantity and of appropriate aerosol size and size characteristics should distribute throughout the lungs and exert a therapeutic response without the need to employ an invasive delivery procedure.